NDT IND RADIOGR FIN210808 - Dam.bakerhughesds

PrefaceTo verify the quality of a product, samples are taken for examination or a non-destructivetest (NDT) is carried out. In particular with fabricated (welded) assemblies, where a highdegree of constructional skill is needed, it is necessary that non-destructive testing iscarried out.Most NDT systems are designed to reveal defects, after which a decision is made as towhether the defect is significant from the point of view of operational safety and/orreliability. Acceptance criteria for weld defects in new constructions have been specifiedin standards.However, NDT is also used for purposes such as the checking of assembled parts, thedevelopment of manufacturing processes, the detection of corrosion or other forms ofdeterioration during maintenance inspections of process installations and in research.There are many methods of NDT, but only a few of these allow the full examination of acomponent. Most only reveal surface-breaking defects.One of the longest established and widely used NDT methods for volumetric examinationis radiography: the use of X-rays or gamma-rays to produce a radiographic image of anobject showing differences in thickness, defects (internal and surface), changes instructure, assembly details etc. Presently, a wide range of industrial radiographic equipment, image forming techniques and examination methods are available. Skill and experience are needed to select the most appropriate method for a particular application.The ultimate choice will be based on various factors such as the location of the object tobe examined, the size and manoeuvrability of the NDT equipment, the existance ofstandards and procedures, the image quality required, the time available for inspectionand last but not least financial considerations.This book gives an overview to conventional industrial radiography, as well as digital(computer-aided) techniques and indicates the factors which need to be considered forselection of the most suitable system and procedures to be followed.At the end of the book a chapter is added describing aspects of radiation safety.1213

1Introduction to industrial radiographyImage forming techniquessourceIn industrial radiography, the usual procedure for producing a radiograph is to have asource of penetrating (ionising) radiation (X-rays or gamma-rays) on one side of theobject to be examined and a detector of the radiation (the film) on the other side as shownin figure 1-1. The energy level of the radiation must be well chosen so that sufficient radiation is transmitted through the object onto the detector.The detector is usually a sheet of photographic film, held in a light-tight envelope or cassette having a very thin front surface that allows the X-rays to pass through easily.Chemicals are needed to develop the image on film, which is why this process is called theclassic or “ wet” process.homogeneousradiationNowadays, different kinds of radiation-sensitive films and detectors not requiring the useof chemicals to produce images, the so-called “dry” process, are used increasingly. Thesetechniques make use of computers, hence the expressions; digital or computer aidedradiography (CR) or genuine (true) digital radiography (DR), see chapter 16.A DR related technique that has been available for many decades is the one in which images are formed directly with the aid of (once computerless) radiation detectors in combination with monitor screens (visual display units: VDU’s), see chapter 17. This is in fact isan early version of DR.These through transmission scanning techniques (known as fluoroscopy) the storage ofimages and image enhancement are continually improved by the gradual implementationof computer technology. Nowadays, there is no longer a clear division between conventional fluoroscopy with the aid of computers and the entirely computer-aided DR. In timeDR will, to some extent, replace conventional fluoroscopy.objectSummarising, the image of radiation intensities transmitted through the component canbe recorded on:cavityThe conventional X-ray film with chemical development, the “ wet” process, or one ofthe following “dry” processes:screensX-ray filmprojection of defect on filmFig. 1-1. Basic set-up for film radiography14 A film with memory phosphors and a work station for digital radiography, calledcomputer-assisted radiography or CR. Flat panel and flat bed detectors and a computer work station for directradiography, called DR. A phosphorescent or fluorescent screen (or similar radiation sensitive medium)and a closed-circuit television (CCTV) camera as in conventional fluoroscopy,an early version of direct radiography.15

By means of radiation detectors, e.g.: crystals, photodiodes or semiconductors in alinear array by which in a series of measurements an image is built up of a movingobject. This method is applied in systems for luggage checks on airports.Assuming the grooves have sharp-machined edges, the images of the grooves couldstill be either sharp or blurred; this is the second factor: image blurring, called imageunsharpness.The source of radiation should be physically small (a few millimetres in diameter), and asX-rays travel in straight lines from the source through the specimen to the film, a sharp“image” is formed of the specimen and discontinuities. This geometric image formation isidentical to the shadow image with a visible light source. The sharpness of the imagedepends, in the same way, on the radiation source diameter and its distance away fromthe surface on which the image is formed.At the limits of image detection it can be shown that contrast and unsharpness are interrelated and detectability depends on both factors.The “classic” film in its light-tight cassette (plastic or paper) is usually placed close behindthe specimen and the X-rays are switched on for an appropriate time (the exposure time)after which the film is taken away and processed photographically, i.e. developed, fixed,washed and dried. In direct radiography (DR), a coherent image is formed directly bymeans of an computerised developing station.The two methods have a negative image in common. Areas where less material (lessabsorption) allows more X-rays to be transmitted to the film or detector will cause increased density. Although there is a difference how the images are formed, the interpretation of the images can be done in exactly the same way. As a result, the DR- technique isreadily accepted.Similarly, in all other image forming systems these three factors are fundamental parameters. In electronic image formation, e.g. digital radiography or scanning systems withCCTV and screens, the factors contrast, sharpness and noise are a measure for the imagequality; pixel size and noise being the (electronic) equivalent of graininess .The “classic” film can be viewed after photochemical treatment (wet process) on a filmviewing screen. Defects or irregularities in the object cause variations in film density(brightness or transparency). The parts of the films which have received more radiationduring exposure – the regions under cavities, for example – appear darker, that is, the filmdensity is higher. Digital radiography gives the same shades of black and white images,but viewing and interpretation is done on a computer screen (VDU).As an image on a photographic film is made up of grains of silver, it has a grainy appearance, dependent on the size and distribution of these silver particles. This granular appearance of the image, called film graininess, can also mask fine details in the image.The three factors: contrast, sharpness and graininess or noise are the fundamental parameters that determine the radiographic image quality. Much of the technique in makinga satisfactory radiograph is related to them and they have an effect on the detectability ofdefects in a specimen.The ability of a radiograph to show detail in the image is called “radiographic sensitivity”.If very small defects can be shown, the radiographic image is said to have a high (good)sensitivity. Usually this sensitivity is measured with artificial “defects” such as wires ordrilled holes. These image quality indicators (IQIs) are described in chapter 13.The quality of the image on the film can be assessed by three factors, namely :1.2.3.ContrastSharpnessGraininessAs an example, consider a specimen having a series of grooves of different depths machined in the surface. The density difference between the image of a groove and the background density on the radiograph is called the image contrast. A certain minimum imagecontrast is required for the groove to become discernible.With increased contrast:a.b.the image of a groove becomes more easily visiblethe image of shallower grooves will gradually also become discernible1617

2Basic propertiesof ionising radiationIn 1895 the physicist Wilhelm Conrad Röntgen discovered a new kind of radiation, whichhe called X-rays. The rays were generated when high energy electrons were suddenlystopped by striking a metal target inside a vacuum tube – the X-ray tube.It was subsequently shown that X-rays are an electromagnetic radiation, just like light,heat and radiowaves.2.1 Wavelengths of electromagnetic radiationThe wavelength lambda (λ) of electromagnetic radiation is expressed in m, cm, mm,micrometer (μm), nanometer (nm) and Ångstrom (1 Å 0.1 nm).Electromagnetic radiationWavelength λm10 km1041 km103100 m10210 m1011m110 cm10-11 cm10-21 mm10-3100 μm10-410 μm10-51 μm10-6Heat-rays, Infra-red rays,microwavesX-ray energy100 nm10-7Visible light and Ultraviolet (UV)100 eV10 nm10-81 keV1 nm10-910 keV0.1 nm10-10100 keV0.01 nm10-111 MeV1 pm10-1210 MeV0.1 pm10-13100 MeV0.01 pm10-14Table 1-2. Overview of wavelength, energy and type of electromagnetic radiation1819TypeX-rays and Gamma-rays(Radiography)

2.3 Gamma-rays (γ-rays)The radiation which is emitted by an X-raytube is heterogeneous, that is, it containsX-rays of a number of wavelengths, in theform of a continuous spectrum with somesuperimposed spectrum lines.See fig. 1-2.Fig. 1-2. X-ray spectrum – intensity/wavelengthdistributionThe small peaks are the characteristic radiation of thetarget materialRadioactivity is the characteristic of certain elements to emit alpha (α), beta (β) orgamma (γ) rays or a combination thereof. Alpha and beta rays consist of electrically charged particles, whereas gamma rays are of an electromagnetic nature.intensity2.2 X-raysGamma rays arise from the disintegration of atomic nuclei within some radioactive substances, also known as isotopes. The energy of gamma-radiation cannot be controlled; itdepends upon the nature of the radioactive substance. Nor is it possible to control itsintensity, since it is impossible to alter the rate of disintegration of a radioactive substance.The shortest wavelength of the spectrum isgiven by the Duane-Hunt formula:1.234kVwavelengthIn which :λ wavelength in nanometers (10 -9 m)kV voltage in kilovoltsThe average shape of the X-ray spectrum is generally the same however not truely identicalfor different X-ray sets; it depends chiefly on the energy range of the electrons striking theX-ray tube target and, therefore, on the voltage waveform of the high-voltage generator.A constant potential (CP) X-ray set will not have the same spectrum as a self-rectified setoperating at the same nominal kV and current. The spectrum shape also depends on theinherent filtration in the X-ray tube window (glass, aluminium, steel or beryllium).Figure 2-2 shows the energy spectrum lines for Selenium75, Cobalt60 and Iridium192.In practical NDT applications, sources (radio active isotopes) are allocated an averagenominal energy value for calculation purposes, see section 5.4. Spectrum componentswith the highest energy levels (keV values) influence radiographic quality the most.relative intensityλmin Unlike X-rays, generated to a continuous spectrum, Gamma-rays are emitted in an isolated line spectrum, i.e. with one or more discrete energies of different intensities.The energy imparted to an electron having a charge e, accelerated by an electrical potential V is (eV) so the energy of the electrons can be quoted in eV, keV, MeV. These sameunits are used to denote the energy of an X-ray spectrum line.The energy of a single wavelength is :Ε h.vλ.v cIn which:E the energy in electronVolt (eV)h Planck’s constantv frequencyc the velocity of electromagnetic radiation, such as light (300,000 km/s)Fig. 2-2. Energy spectrum (lines) for Se75, Ir192 and Co60The heterogeneous X-rays emitted by an X-ray tube do not however have a singlewavelength, but a spectrum, so it would be misleading to describe the X-rays as (say)120 keV X-rays. By convention therefore, the ‘e’- in keV- is omitted and the X-raysdescribed as 120 kV, which is the peak value of the spectrum.2021energy (keV)

2.4 Main properties of X-rays and γ-rays2.6 Absorption and scatteringX-rays and γ-rays have the following properties in common:The reduction in radiation intensity on penetrating a material is determined by thefollowing reactions :1. invisibility; they cannot be perceived by the senses2. they travel in straight lines and at the speed of light3. they cannot be deflected by means of a lens or prism, although their path can be bent(diffracted) by a crystalline grid4. they can pass through matter and are partly absorbed in transmission5. they are ionising, that is, they liberate electrons in matter6. they can impair or destroy living cells2.5 Radiation energy-hardnessRadiation hardness (beam quality) depends on wavelength. Radiation is called hardwhen its wavelength is small and soft when its wavelength is long. In industry the qualityof the X-ray tube ranges from very soft to ultra hard. The beam quality is related to a tubevoltage (kV) range, or keV for isotopes.The first two columns of table 2-2 below indicate the relationship hardness/tube voltagerange applied in NDT. The third column gives the related qualification of the radiationeffect, i.e. half-value thickness (HVT), described in detail in section 2.9.Radiation qualityTube voltageGlobal half-valueHardnessVery softthickness for steel (mm)Less than 20 kVSoft20 – 60 kVFairly soft60 – 150 kV0.5-2Hard150 – 300 kV2-7Very hard300 – 3000 kV7-20Ultra hardmore than 3000 kV 20Table 2-2. Comparative values of radiation quality (hardness) against tube voltage.221. Photoelectric effect2. Compton effect3. Pair productionWhich of these reactions will predominate depends on the energy of the incidentradiation and the material irradiated.Photoelectric effectWhen X-rays of relatively lowenergy pass through a materialand a photon collides with anatom of this material, the totalenergy of this photon can beused to eject an electron from theinner shells of the atom, as figure3-2 illustrates. This phenomenonis called the photoelectric effectand occurs in the object, in thefilm and in any filters used.Compton effectWith higher X-ray energies (100keV to 10 MeV), the interactionof photons with free or weaklybonded electrons of the outeratom layers causes part of theenergy to be transferred to theseelectrons which are then ejected,as illustrated in figure 4-2. At thesame time the photons will bedeflected from the initial angleof incidence and emerge fromthe collision as radiation of reduced energy, scattered in all directions including backward, knownas “backscatter”, see section 17.6.In this energy band, the absorption of radiation is mainly due tothe Compton effect and less so tothe photoelectric effect.incidentX-raysejectedelectronFig. 3-2. Photoelectric effectejectedelectronX-ray100keV - 10 MeVscatteredradiationFig. 4-2. Compton effect23

2.7 Penetrating powerejectedelectronThe penetrating power of X-radiation increases with the energy (hardness).The relationship of energy and penetrating power is complex as a result of the variousmechanisms that cause radiation absorption. When monochromatic ( homogeneous single wave length) radiation with an intensity Io passes through matter, the relativeintensity reduction ΔI/Io is proportional to the thickness Δt. The total linear absorptioncoefficient (μ) consisting of the three components described in section 2.6 is defined bythe following formula:ΔI μ.ΔtIoejectedpositronFig. 5-2. Pair productionTotal absorption/attenuationThe total linear absorption orattenuation of X-rays is a combination of the three absorptionprocesses described above, inwhich the primary X-ray energychanges to a lower form of energy. Secondary X-ray energy arrises of a different wavelength anda different direction of travel.Some of this secondary (scattered) radiation does not contribute to radiographic image formingand may cause loss of imagequality through blurring or fog.The contribution of the variouscauses of X-ray absorption to thetotal linear absorption coefficient (μ) for steel plotted againstradiation energy, are shown infigure 6-2.Expressed differently:In which:Io intensity at material entryI intensity at material exitμ total absorption coefficientI Io. e-μtt thicknesse logarithm: 2.718Figure 7-2 shows the resulting radiationintensity (logarithmic) as a function ofincreased material thickness, for soft andhard homogeneous radiation.When radiation is heterogeneous thegraphs are not straight, see figure 7-2, butslightly curved as in figure 8-2.Fig. 6-2 Absorption coefficient for steel plotted against radiation energyPE Photoelectric effectC Compton effectPP Pair productionFig. 7-2. Intensity of homogeneousradiation as function of increasingthicknessintensityX-ray 1 MeVThe slope of the curves becomes graduallyshallower (because of selective absorptionof the softer radiation) until it reaches theso-called “point-of-homogeneity”.After passing this point the coefficient ofabsorption remains virtually unchanged, asif the radiation had become homogeneous.The position of the point of homogeneityvaries with the nature of the material irradiated. The graph shows that with increasingmaterial thickness, softer radiation is filtered out, more than hard radiation.This effect is called “hardening”.hard radiation,high tube voltagesoft radiation,low tube voltagepenetrated material thicknessFig. 8-2. Intensity of heterogeneousradiation as function of increasingthickness hard radiationhard radiationintesityPair productionThe formation of ion pairs, seefigure 5-2, only occurs at veryhigh energy levels (above 1 MeV).High-energy photons can causean interaction with the nucleusof the atom involved in the collision. The energy of the photon ishere converted tot an electron(e-)and a positron (e ).soft radiationpoints of homogeneitypenetrated material thickness2425

Table 2-2 shows the average HVT-values for steel, table 3-2 shows the values for lead.2.8 Filtering (hardening)All materials, for example a metal layer between the radiation source and the film, causeabsorption and filtering. The position of the metal layer plays an important role in theeffect it has. A metal layer in front of the object will “harden” the radiation because it filters out the soft radiation. The degree of hardening depends on the type and the thicknessof the material. This phenomenon is used to reduce excessive contrast (variation in density)

number of computer-aided NDT techniques which have entered the market in recent years. In 2007 a new edition in the English language was published by GE Inspection Technologies. That edition was compiled by Mr. J.A. de Raad, NDT-expert and consultant, who has a considerable number of publications on the subje